1. Field of the Invention
The present invention relates to a superconducting high-frequency device, and more particularly, to a dual-mode superconducting device applied to front end devices, such as transmission filters or transmission antennas, in mobile communications systems or broadcast systems.
2. Description of the Related Art
Along with recent spread and progress of mobile (cellular) phones, high-rate high-capacity transmission techniques are becoming indispensable. Application of superconductors to base station filters for mobile communications is greatly expected, being promised as providing low loss and high Q value resonance, because superconductors have very small surface resistance as compared with ordinary electric conductors, even at a high-frequency region.
For example, as illustrated in FIG. 1C, the RF signal received at the antenna (ANT) 151 is subjected to baseband processing at the baseband processing unit 156, after having passed through the bandpass filter (BPF) 152R, the low-noise amp (LNA) 153, the down converter (D/C) 154, and the demodulator (DEMOD) 155.
In the transmission system, the signal processed by the baseband processing unit 156 passes through the modulator (MOD) 157, the up converter (U/C) 158, the high-power amp (HPA) 159, and the bandpass filter (BPF) 152T, and is finally transmitted from the antenna 151.
When applying a superconductive filter as the receiving-end bandpass filter 152R, a steep frequency cutoff characteristic can be expected with less transmission loss. On the other hand, application to the transmission-end bandpass filter 152T leads to the effect for removing distortion caused by the high-power amp 159. However, the transmission end requires high power to transmit a radio signal, and therefore, simultaneous pursuit of compactness and a satisfactory power characteristic is the present issue.
Conventionally, a resonator is provided with a superconducting filter pattern (signal layer) 102 of a hairpin type illustrated in FIG. 1A, or a straight-line type illustrated in FIG. 1B. See, for example, JP 2001-308603A and JP 3-194979A. The bottom of a dielectric substrate 101 is covered with a superconducting ground film (blanket film) 104, while the top face is furnished with a hairpin or straight-line superconducting filter pattern 102 and a feeder 103.
Conventional filters with the above-described microstrip structure have a problem in that transmission loss increases especially at the transmission end when high RF power is input. This is because a high-frequency wave, such as a microwave, is likely to concentrate on the edge of the conductor pattern, causing concentration of electric current on the edge or the corner of the microstrip line, and because the electric current density exceeds the critical current density of the superconductor.
To overcome this problem, a disk pattern has been proposed to reduce concentration of electric current, as illustrated in FIG. 2A. In this example, a superconducting disk pattern 112 with fewer corners or edges is formed on the dielectric substrate 101 in order to realize a high power response as the transmission filter.
When the filter pattern is formed as a TM11 mode disk resonator, the electric current flows uniformly along the symmetric arcs with respect to the diameter of the disk, as illustrated in FIG. 2B. The magnetic field points in a direction perpendicular to the electric current.
However, a multistage filter or a multistage array antenna with several disk resonators arranged in it has a drawback of increasing the device size.
Then, a superconducting disk pattern 122 with a notch 125 formed on a portion of the circumference of the disk is proposed. By forming the notch 125, the degeneracy of the mutually orthogonal electric and magnetic fields of the mode is lifted to separate the resonate frequency so as to allow the resonator to function as a dual-mode filter. In the example shown in FIG. 3, two types of resonance at lower frequency f1 (with electric current flow in direction A) and higher frequency f2 (with electric current flow in direction B) with respect to the center frequency f0 are generated.
However, the notch 125 formed in the superconducting disk pattern 122 causes the electric current to concentrate on the corners of the notch 125 on the lower frequency f1 side, as illustrated in FIG. 3, resulting in exceeding the maximum electric current density of the basic disk resonator without a notch. In FIG. 3, concentration of electric current occurs in the shaded areas indicated by the arrows. Electric current concentration is conspicuous especially at the bottom edge and the bottom corners of the square-shaped notch 125. In contrast, the area along the circumference of the superconducting disk pattern 122 has less electric current concentration. Frequencies f1 and f2 are 45 degrees out of phase at the maximum electric current density.
Electric current concentration on the corners and edges of the notch 125 will cause a decrease of the maximum allowable power and an increase of distortion in the bandpass filter or the antenna using a superconducting resonator.
Concerning a microstrip type high-frequency transmission line, it is proposed to form a straight groove along the edge of the electrode formed on the dielectric substrate to disperse the electric current concentration on the edge. See, for example, JP 11-177310.